1. Field of the Invention
The present invention concerns an electronic variable speed drive including, in a cabinet having an opening door, a power system connected to an AC line voltage and delivering a voltage under the control of solid state power switches, and an electronic control system controlling the solid state switches.
2. Description of the Prior Art
Electronic variable speed drives are routinely used in association with DC and AC motors to drive the motor at variable speed.
Variable speed drives known as frequency converters for controlling the speed of AC motors derive from the single-phase or three-phase AC line voltage an AC voltage whose rms value and frequency vary according to a predetermined law. These frequency converters essentially comprise a rectifier supplying a DC voltage to a smoothing (filter) capacitor (AC-DC converter) and a power transistor inverter (DC-AC converter) energized by the filtered DC voltage,
The rectifier is a rectifier bridge connected to a single-phase or three-phase AC line voltage. The inverter is connected to the phase windings of the motor and produces an AC voltage from the DC voltage supplied by the rectifier.
The power transistors are controlled by pulse width modulation (PWM) to generate a voltage as close as possible to a true sinusoid.
Patent EP-A-0 103 412 and patent DE-A-3 412 510 describe electronic variable speed drives. These prior art variable speed drives do not provide sufficient protection.
An object of the present invention is to provide a variable speed drive in which the protection arrangements are designed to allow maintenance or repair personnel access to functional parts of the variable speed drive while the latter is operating. Optional circuit boards can also be fitted or removed in complete safety, whether the product is live or not. The protection arrangements are designed to prevent metal and conductive dust particles falling vertically into the unit disturbing its operation, to guarantee protection of personnel and to enable dissipation of heat from internal components of the variable speed drive which operate at raised temperature. The protection category of the cabinet is IP30 with the door closed and IP20 with the door open.